Wastewater modeling of Cr (VI) adsorption by sulfonated lignin of pine cones: equilibrium, kinetics, and isotherm study

Abstract Water is vital to life, so preserving it from pollution is an important issue. Chromium, in its various oxidation states, poses a significant threat to water contamination. This study analyzes the sulfonation process, which improves the Cr (VI) adsorption capability of lignin (a bio-adsorbent generated from pine cones), to produce sulfonated lignin (SL). The optimal pH for Cr (VI) adsorption was 2. Within the first ten minutes, uptake was rapid; however, it slowed thereafter and eventually reached equilibrium after 30 min. The relationship between the initial and equilibrium concentrations of Cr (VI) was linear. The percentage of Cr (VI) removal increased as the SL dosage increased. The adsorption on SL was better explained by the Lagergren pseudo-second-order kinetic model than by the Elovich or intraparticle diffusion models. The adsorption data were better correlated by the linear form of the Langmuir-2 isotherm model than by the Freundlich and Tempkin models. The maximum monolayer sorption capacity (Qm) from the Langmuir isotherm model was determined to be 34.714 mg/g. The high monolayer capacity of SL implies that it is a highly effective adsorbent for Cr (VI). SL is a cost-effective, eco-friendly, and promising adsorbent with outstanding potential for removing Cr(VI) contaminants from polluted water. Graphical abstract